According to textbooks, many past climate changes, including the ice ages, closely followed the rhythmic nodding and wobbling of Earth's spin axis and the periodic stretching of its orbit. The astronomical pacing implies clocklike regularity, but climate records in ice cores and deep-sea sediments don't always show that. In tomorrow's issue of Science[2], researchers attempt to explain why climate change may not always exactly follow the astronomical clock: An interaction between two long-term cycles can produce a new pattern, they say, in much the same way as FM radio signals are generated.

Cycles of 23,000 and 41,000 years in the climate record match up precisely with Earth's wobbling and nodding, but the ice ages don't keep to the 100,000-year schedule that should be set by the periodic elongation of Earth's orbit. Some ice ages have come at intervals of as much as 120,000 years or as little as 80,000 years, says geophysicist José Rial of the University of North Carolina, Chapel Hill. But Rial noticed an underlying regularity: A complete cycle from quicker cycles to slower ones took about 400,000 years, suggesting that yet another astronomical cycle--a second, 413,000-year cycle in orbital elongation superimposed on the shorter one--might be modulating the frequency of the 100,000-year cycle, the same way broadcasters "frequency modulate" a carrier signal to produce FM radio broadcasts.

To test his idea, Rial calculated how a 413,000-year signal should modulate a 100,000-year one and checked the climate record to see how it matched the simulation. He found that the frequency of the 100,000-year cycle has risen and fallen in time with the longer modulating cycle, matching the calculation. As in an FM broadcast, the modulating signal itself failed to show up in the climate record but left its fingerprints in pairs of "sideband" signals that have frequencies just above and below the carrier frequency. Rial found exactly the predicted pattern of sidebands in the climate record.

Other climate specialists are impressed. "I like very much the ideas of Rial," says paleoclimatologist André Berger of the Catholic University of Louvain in Belgium. Geodynamicist Bruce Bills of the Scripps Institution of Oceanography in La Jolla, California, calls the fit "intriguing." But, he says, "it would be even better if you could point to an obvious physical mechanism that would explain why the system works that way." Rial doesn't have such a mechanism yet, but he says that the longer cycle of sunlight changes must somehow affect how fast the ice sheets build up or collapse.